The invention relates to a fuel cell system and more particularly to a fuel cell system capable of exhausting reacted gases therein.
A fuel cell is a power generating device that transforms chemical energy to electrical energy. Fuel cells emits lower or zero pollutants, are quiet, and provide higher energy density and higher energy transforming efficiency compared to conventional power generating techniques. Fuel cells are considered to be a clean energy source suitable for future applications such as portable electronic devices, household electric power generating system, transportation, military equipment, and industrial and large-scale electric power generating systems.
There are substantially five types of fuel cells each utilizing a different electrolyte. An alkaline fuel cell (AFC) utilizes potassium hydroxide as an electrolyte. A phosphoric acid fuel cell (PAFC) utilizes a phosphoric acid solution as an electrolyte. A molten carbonate fuel cell (MCFC) utilizes melted carbonic acid containing compounds as an electrolyte. A solid oxide fuel cell (SOFC) utilizes zirconium oxide as an electrolyte. A proton exchange membrane fuel cell (PEMFC), including the so-called direct methanol fuel cell (DMFC), utilizes methanol as a fuel without forming hydrogen in advance.
Because the DMFC uses liquid or gaseous methanol as a fuel supply source directly and does not require recombining of other materials such as methanol, gasoline or natural gases to form hydrogen for generation of electric power. Moreover, the DMFC generates power at a lower temperature and with a fuel composition with less danger. Thus DMFCs are suitable for application in portable electronic devices.
A conventional DMFC is mainly formed of a membrane electrode assembly (MEA), an anode current collector and a cathode current collector. The MEA comprises a proton exchange membrane, an anode catalyst layer, a cathode catalyst layer, an anode gas diffusion layer, and a cathode gas diffusion layer. The anode catalyst layer and the cathode catalyst layer are disposed on each side of the proton exchange membrane respectively. The anode gas diffusion layer and the cathode gas diffusion layer are disposed on the anode catalyst layer and the cathode catalyst layer respectively. The anode current collector and the cathode current collector are disposed over the anode gas diffusion layer and the cathode gas diffusion layer respectively. Reactions in the DMFC occur according to the following formulas (1) to (3).At the anode: CH3OH+H20→CO2+6H++6e−  (1)At the cathode: (3/2)O2+6H++6e−→3H2O  (2)Overall reaction: CH3OH+(3/2)O2→CO2+2H2O  (3)
According to formula (1), CO2 is generated at the anode during the DMFC operation and must be properly exhausted to prevent accumulation of gaseous pressure at the anode. The CO2 is easily separated from the liquid when the anode uses liquid fuel. Separation of CO2 from gaseous fuel in the fuel cell when using gaseous fuel is however difficult. Thus, gaseous fuel may be also exhausted from the fuel cell, resulting in gaseous fuel loss and affecting fuel cell conversion efficiency. An anode moisture-keeping layer maybe provided for forcing the vaporized methanol to pass through the anode moisture-keeping layer and into the anode gas diffusion layer. Moisture at the anode may, however, also pass through the anode moisture-keeping layer and diffuse into a fuel storage tank thereof, causing reduction of a fuel concentration therein, as disclosed in WO patent application 2005/112172A1. In addition, as disclosed in WO patent application 2006/040961A1, a CO2 exhaust can be formed at a sidewall of a vaporized fuel reserve compartment and the vaporized methanol concentration in the vaporized fuel reserve compartment is slightly less than the fuel storage tank, thereby causing more vaporized methanol to be exhausted with the CO2, polluting the environment, and reducing fuel conversion efficiency.